1. Field of the Invention
This invention relates to structural beam members and particularly to pre-cambered roll formed beam members.
2. Prior Art
The advantages of certain forms of cambered and prestressed beams have been recognized in the construction industry for many years. Depending upon the particular use to which a beam is put, it may take various shapes and be formed in various ways. Illustrative of such beams, their form and formation are U.S. Pat. Nos. 3,300,839 to R. D. Lichti; 2,986,246 to R. W. Lester; and 3,010,272 to G. W. Setzer.
Lichti shows a method of making beams in which a camber is imparted to an I or H beam during its construction. The upper and lower flanges and the center corregated web member of the I beam are held in a curved position by a jig assembly and welded together.
Setzer discloses a method for improving structural members, such as beams, in which the structural members are pre-loaded by permanently imparting reverse loads to the members to resist the loads which are to be imposed in use. The member is improved by adding welding metal to the top flange, the bottom flange or both, in areas where the pre-load is desired. For example, welding metal is applied along the central portion of the lower flange of the beam, as shown in FIGS. 1 and 5 (see elements 4 and 14 respectively). The metal is applied by a conventional welding operation which causes some heating of the beam in and adjacent the area where the molten welding material is laid. The degree of heating of the beam, however, is much less than the heat of the molten welding material and, consequently, the welding metal will shrink in cooling to a greater degree than the beam. As the welding metal commonly used has a greater tensile strength than the metal of the beam, the shrinking welding material will draw the material of the beam with it, so that the portion of the beam beneath the welding material will be placed in compression. By proper placement and spacing of a number of such areas of welding material (e.g., as at 5 in FIG. 1) the beam may be cambered slightly at its ends and thereby "preloaded." This preloading may be applied to beams, columns, trusses, etc.
Lester discloses a preloaded, prestressed beam structure. However, the prestressing procedure is applied at the time of, or following, installation, not as part of the manufacturing process of the beam. Moreover, to cause the cambering, external devices are required to deform the beam (e.g., the screw-adjustable prestressing members 26 and 28 and a pair of screw adjustable preloading members 30 and 32).